Selecting Liquid Level Detectors For Tanks

How much liquid is in the tank? How much did we use today? Is it time to refill? These are common questions around a typical plant. There are many ways to determine how much liquid is in a vessel.

By Joseph L. Foszcz

02/01/1998

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How much liquid is in the tank? How much did we use today? Is it time to refill? These are common questions around a typical plant.

There are many ways to determine how much liquid is in a vessel. In some cases it may be necessary or desirable to combine two or more methods because of tank design or liquid properties. The process should consider the points listed in the "Selection factors" table.

At first, the choice of a level detector may seem confusing, but there are only two functions to initially consider: point and continuous level sensing. Point level sensors detect the liquid at a fixed point in a vessel. They can sense when a tank is full, empty, or at any preset position in between. Continuous level sensors provide an indication of the liquid level no matter where it is. The manufacturers box at the end of this article shows that some level indicators can perform both functions.

Continuous level sensors provide a signal to a readout that indicates the amount of liquid in a tank at any time. They are usually complex and accurate, which is reflected in their relatively high cost. Because of their design, this type of sensor can be easily set to maintain any tank level by actuating pumps or valves.

Capacitance

Capacitance transmitters or probes are long and cylindrical, usually reach to the bottom of the tank, are top-mounted, and require overhead clearance. Impedance between the probe and metallic tank wall is an indication of liquid level. Designs are available for nonmetallic tanks. Compensation can be made if tank walls are not straight.

Because they contain no moving parts, capacitance transmitters are durable and suited for use with viscous liquids and where there is turbulence. Wide pressure and temperature ranges have little effect on the performance of these instruments. Accuracy is affected if the liquid wets or adheres to the probe.

Conductive

Conductive transmitters are applicable for single-point measurement with a conductive fluid. Pairs of probes are used, and an electric path is provided through the conductive fluid when both probes are covered. Current flow is used to provide a signal at that level. Multipoint measurements can be made to detect different levels (Fig. 1). This method is inexpensive and the equipment fairly rugged. Precautions should be taken because these devices are not intrinsically safe.

Differences in conductivity can also be used to detect interfaces. When the liquid is water, the sensor conducts a small current. When the liquid is hydrocarbon based, conduction is virtually zero. In a tank containing both, a change in conductivity indicates the location of the interface.

Circuitry can eliminate errors caused by conductive coatings adhering to the probes. Designs are also available to eliminate the jitter in a display caused by agitated fluids. There are no moving parts and the probes work with fluids that have low density, high viscosity, or contain large quantities of suspended particles.

Float

Flotation is the simplest level measuring method and makes use of a float that follows the liquid level in a tank. The position of the float is magnetically coupled to switches that are used to indicate the level at any point (Fig. 2).

Float switches can provide continuous level, point level, density, or interface measurements. They are generally used in clean fluids over a wide temperature and pressure range. These devices are unaffected by agitation or turbulence and are immune to changing dielectric properties or foam.

Nuclear

Continuous nuclear level detection is typically used where most other technologies are unsuccessful. Different radioactive isotopes are used, based on the penetrating power needed to pass through the tank. Radiation from the source is detected on the other side of the tank. Its strength indicates the level of the fluid. Point, continuous, and interface measurements can be made (Fig. 3).

The devices are noninvasive, making them ideal for use with corrosive or viscous liquids or with extreme process conditions. Readings are affected by density changes, but not by agitation.

Nuclear level detection has some drawbacks. One is high cost, up to four times that of other technologies. Others are the probable requirement for licenses, approvals, and periodic inspections; and the difficulty and expense of disposing of spent radiation materials. Another factor to consider is that the radiation symbol found on these devices can cause concern to plant personnel.

Optical

Optical level detectors are used for point measurement and have a light source and photodetector mounted in one housing. Light from an LED is directed to a prism. If liquid is not present, the light is reflected back to the detector. When liquid is present, light is dispersed and does not trip the detector (Fig. 4).

Optic sensors are efficient and can be mounted in any position. They are normally used with clear or translucent liquids. Condensation on the sensing tip does not affect detection. Units are self-contained and have no moving parts. They are limited to applications with temperatures up to 185 F and pressures up to 145 psig.

Pressure

Pressure transmitters are the most commonly used devices for measuring liquid levels, but they do not directly measure level. They measure pressure caused by the height, or head, of liquid in a tank. The head multiplied by the liquid density translates into level height. Fluid density must be stable if readings are to be accurate. A second transmitter may be required if density changes.

A primary benefit of these transmitters is simple installation, sometimes in a drain line. They work best with clean liquids and should not be used with liquids that solidify as their concentrations increase.

If a tank is not pressurized, the transmitter is called a gauge pressure transmitter. Pressurized tanks require a differential pressure transmitter whose low pressure side is referenced to the pressure above the fluid (Fig. 5). Either type of device can be used on open tanks or those vented to the atmosphere.

Radar

Radar level measurement is noncontacting, noninvasive, and not sound dependent. It can be used for continuous or point measurement. Level is measured independently of changes in density. The device works well in clean, aerated, solids-laden, viscous, or corrosive fluids. It cuts through foam layers and is unaffected by changes in vapor space.

Radar signals or high-frequency, electromagnetic waves are sent from the transmitter to the surface of the liquid and reflected back to a receiver. The time it takes to return to the receiver is proportional to the level.

Radar can be used from -40 to 800 F and from vacuum to 800 psi. Measurement is minimally affected by air temperature, dust, or particles. Radar cannot be used to measure interfaces. Although an expensive technology, it is highly accurate.

Thermal

The thermal element method is used for point measurements and depends on the thermal conductivity of the fluid being higher than its vapor. When the thermal element comes in contact with liquid, the rate of heat transfer increases, causing its resistance to increase. Equipment used with this method is rugged and not affected by changes in ambient conditions. The method is not intrinsically safe and may cause changes in the liquid.

Advantages of the thermal method include being suitable for setting high and low limits in dirty, noncoating fluids over a known range of specific gravities. It works well in turbulent and foaming liquids and slurries.

Ultrasonic

Ultrasonic devices are capable of making both point and continuous level measurements independent of changes in density and dielectric constants. They work well in clean, solids-laden, viscous, or corrosive fluids; and some slurries and aerated liquids.

This method uses an oscillator to excite a sensor and send sound pulses through the air. For point measurements the signal is picked up by another sensor as long as a transmission path is available. When the liquid level rises, the path is interrupted, indicating the level. Another method is to allow the liquid to damp the sensor vibration. Dampening is detected to indicate the level.

Continuous measurements are accomplished by using intermittent transmission and measuring the time taken for the signal to return to the sensor (Fig. 6). The return signal can be affected by dust, steam, surface turbulence, foam, ambient noise, and high temperatures and pressures. Heavy vapors from the liquid itself, such as hydrocarbons, tend to stratify and have a pronounced affect on accuracy.

Successful measurement depends on the transmitter being mounted in the correct position in the vessel to eliminate or reduce signals from internal components such as ladders, agitators, and floats. If the level drops below these components, they can provide a false signal.

Vibrator

A vibrating, tuning-fork sensor is a point level detector. A pair of tines vibrate at a fixed, high frequency. When the liquid covers the tines, the vibration frequency changes, signaling the liquid level.

Tuning-fork sensors are suited to liquids that frequently change in composition. Calibration of the fork remains constant with use over a wide range of noncoating liquids, from lubricating oils to hydraulic fluids, where viscosities can range up to 5000 cSt. They operate at temperatures up to 270 F and pressures up to 650 psig.

The level of practically any liquid can be measured. If one technology doesn't work, there are one or more others that will. Whether a point or continuous measurement must be made, some devices can do one or both. The only limitations are how important is the measurement and how expensive it is.

Plant Engineering magazine extends its appreciation to all respondents to the questionnaire for their assistance in the preparation of this article, and particularly to the Bindicator Co. and Flowline, Inc.